Magnesium transport across the basolateral plasma membrane of the fish enterocyte

In tilapia (Oreochromis mossambicus) intestine, Mg2+ transport across the epithelium involves a transcellular, Na+- and Na+/K+-ATPase dependent pathway. In our search for the Mg2+ extrusion mechanism of the basolateral compartment of the enterocyte, we could exclude Na+/Mg2+ antiport or ATP-driven transport. Evidence is provided, however, that Mg2+ movement across the membrane is coupled to anion transport. In basolateral plasma membrane vesicles, an inwardly directed Cl- gradient stimulated Mg2+ uptake (as followed with the radionuclide 27Mg) twofold. As Cl--stimulated uptake was inhibited by the detergent saponin and by the ionophore A23187, Mg2+ may be accumulated intravesicularly above chemical equilibrium. Valinomycin did not affect uptake, suggesting that electroneutral symport activity occurred. The involvement of anion coupled transport was further indicated by the inhibition of Mg2+ uptake by the stilbene derivative, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid. Kinetic analyses of the Cl--stimulated Mg2+ uptake yielded a Km (Mg2+) of 6.08 +/- 1.29 mmol . l-1 and a Km (Cl-) of 26.5 +/- 6.5 mmol . l-1, compatible with transport activity at intracellular Mg2+- and Cl--levels. We propose that Mg2+ absorption in the tilapia intestine involves an electrically neutral anion symport mechanism.